Separating carbon dioxide, hydrogen sulfide and hydrocarbons in streams containing a large percent of carbon dioxide, e.g., as high as 80 or 90 mole%, has become an area of great interest. This interest comes primarily from two areas. One is the recovery of hydrocarbons from naturally occurring gases which were previously thought uneconomical to produce. The second area relates to the increased activity in enhanced oil recovery (EOR). In particular, the use of carbon dioxide for miscible flood is gaining momentum and the associated gas from wells which have been stimulated by this method must be processed at well heads or satellite treating plants.
It is well known that appropriate physical solvents such as propylene carbonate and dimethyl ether of polyethyleneglycol are selective toward acid gas components. A drawback of using physical solvents, however, is that although suitable for bulk CO.sub.2 removal, they are not attractive for the above operations because they co-absorb C.sub.3 and heavier hydrocarbons.
Suitable chemical solvents which will react with acid gas components are aqueous solutions of potassium carbonates and of amines such as monoethanolomine, diethanolamine, etc. Processes employing these chemical solvents are generally too energy intensive and costly at high CO.sub.2 levels.
Several distillation processes have been developed for separating acid gases from hydrocarbons. Two such processes are described in U.S. Pat. Nos. 4,318,723 and 4,370,156. Distillation is effective for bulk CO.sub.2 recovery, however, the process must be carried out at cryogenic temperatures in which refrigeration must be provided either by an external means or by expanding part of the compressed gas. Additionally, a large amount of lean oil must be circulated to the tops of the distillation column to prevent CO.sub.2 freezing and CO.sub.2 /ethane azeotrope formation.
Membrane separation units have also been used to separate acid gases from hydrocarbon streams. U.S. Pat. No. 4,130,403 discloses a method wherein a stream from which selected components are to be separated is brought into contact with one side of a permeable membrane. The membrane used, such as a cellulose ester membrane, has permeability constants for H.sub.2 S or CO.sub.2 of approximately at least 10.sup.-8 (cc)(cm)/(sec)(cm.sup.2)(cmHg). Upon contact, the more permeable components of the feed gas will pass through the membrane to a much greater extent than other components, thereby effecting the desired separation. Following contact with the membrane, both the residue stream and the permeate gas stream are separately removed from contact with the membrane.
U.S. Pat. No. 4,374,657 discloses a process for separating acid gases from hydrocarbons by first separating methane from the hydrocarbon stream by a separation method such as low temperature distillation to produce a substantially methane-free hydrocarbon stream containing acid gases, ethane and heavier hydrocarbon components. The substantially methane-free hydrocarbon stream is subsequently passed through a semipermeable membrane system to separate the acid gases from the ethane and heavier hydrocarbons.
Various gas-hydrocarbon separation processes were described by C. S. Goddin in "Comparison of Processes for Treating Gases with High CO.sub.2 Content" Annual GPA Convention, Mar. 15-17, 1982. One such process involves using a membrane such as cellulose acetate or polysulfone, to produce a CO.sub.2 permeate containing not more than 5 mole% hydrocarbons and a hydrocarbon effluent containing 20 mole% CO.sub.2. The latter stream is sent to a conventional DEA unit for final removal of CO.sub.2 and H.sub.2 S. Both the acid gas from the DEA stripper and the sour CO.sub.2 permeate are compressed and sent to a Selexol unit for sweetening.
U.S. Pat. No. 4,466,946 describes a method of removing carbon dioxide from a gas stream containing CO.sub.2 and hydrocarbons. The gas stream is treated to prevent hydrocarbons from condensing out during CO.sub.2 removal. Initial separation can be accomplished by selective permeation of CO.sub.2 across a differentially permeable membrane.
Additional processes for separating acid gases from hydrocarbons using semipermeable membranes are described in U.S. Pat. Nos. 4,264,338 and 4,435,191.